Known to the art are voltage converters (or, in a similar way, regulators or power supplies) that have a galvanic insulation between an input voltage and a regulated output voltage, having a desired value, in which the galvanic insulation is obtained via a transformer having a primary side receiving the input voltage, and a secondary side supplying the regulated output voltage. Generally used are two techniques for controlling these voltage converters, which envisage a feedback either on the secondary side or on the primary side of the transformer. In the first case, a feedback voltage is taken directly on a secondary winding of the transformer, in parallel to the output, and sent to a regulation circuit via an optocoupler device so as to maintain the galvanic insulation. In the second case, the feedback voltage is taken generally on an auxiliary winding, purposely provided on the primary side of the transformer. The feedback on the primary side makes it possible to avoid the use of external isolation devices (for example, additional optocouplers or transformers), but may entail higher levels of consumption and hence a degradation in the regulation efficiency.
A wide range of control techniques has been proposed for implementing an efficient voltage regulation with feedback from the primary winding, but so far none of these has proven altogether satisfactory.
In particular, it has been proposed to use a purposely provided sample-and-hold device for sampling the feedback voltage on the auxiliary winding at the end of demagnetization of the transformer, i.e., when the value of this voltage corresponds to the value of the output voltage, constituting, in a know way, a faithful replica thereof.
In detail, and as is shown in FIG. 1, a voltage converter 1, of a flyback isolated type with control of the peak current and feedback on the primary winding, has a first input terminal IN1 and a second input terminal IN2, which are designed to receive an input voltage Vin, for example, from a voltage generator 2, and a first output terminal OUT1 and a second output terminal OUT2, between which an output capacitor 3 is coupled and an output voltage Vout with regulated value is present. The voltage converter 1 supplies to a load an output current Iout.
The voltage converter 1 comprises a transformer 4, having a primary side and a secondary side, which is electrically isolated from the primary side, and having a primary winding 5, a secondary winding 6, and an auxiliary winding 7 (the latter positioned on the primary side of the transformer 4). For example, the transformer 4 has a turn ratio N between the primary winding 5 and the secondary winding 6, and a unit turn ratio between the secondary winding 6 and the auxiliary winding 7 (N:1:1). The primary winding 5 has a first terminal, which is coupled to the first input terminal IN1, and a second terminal, which is coupled to a control switch 8, which can be actuated for controlling PWM operation of the voltage converter 1. The secondary winding 6 has a respective first terminal, which is coupled to the first output terminal OUT1, via the interposition of a first rectifier diode 9, and a respective second terminal, which is coupled to the second output terminal OUT2. The auxiliary winding 7 has a respective first terminal, present on which is an auxiliary voltage Vaus and which is coupled to a resistive divider 10, and a respective second terminal, which is coupled to the reference potential.
The control switch 8, for example a power MOS transistor, has a first conduction terminal, which is coupled to the primary winding 5, a second conduction terminal, which is coupled to the reference potential, via the interposition of a sense resistor 11, and a control terminal, which is coupled to a control circuit 12, designed to control PWM operation of the voltage converter 1.
The resistive divider 10 comprises a first resistor 13 and a second resistor 14, which are coupled in series between the first terminal of the auxiliary winding 7 and the reference potential and define an intermediate node 15 on which a feedback signal Vfb is present.
The voltage converter 1 further comprises a self-supply capacitor 16 coupled to the auxiliary winding 6 via the interposition of a second rectifier diode 17 and is designed to supply, in a known way, a self-supply voltage Vcc to the control circuit 12 during the demagnetization phase of the transformer 4.
In detail, the control circuit 12 has a first input 12a, which is coupled to the intermediate node 15 and receives the feedback signal Vfb, a second input 12b which is coupled to the sense resistor 11 and receives a sense voltage Vs, and an output 12c, which is coupled to the control terminal of the control switch 8 and supplies a driving signal PW.
The control circuit 12 comprises: a sampling stage 20, which is coupled to the first input 12a and supplies at output a sampled signal Vcam, which is the result of the sample and hold (for example, performed at each switching cycle) of the feedback signal Vfb at the end of the demagnetization phase; an error-amplifier stage 22, having a first input terminal, which is coupled to the output of the sampling stage 20 and receives the sampled signal Vcam, a second input terminal, which is coupled to a reference generator 23 and receives a reference voltage Vref, the value of which is a function of a desired value of the regulated output voltage Vout, and an output terminal, which is coupled to an external compensation network 24 (represented schematically in FIG. 1 by a load impedance). A voltage control signal Vcon is consequently present on the output terminal of the error-amplifier stage 22.
The control circuit 12 further comprises a controller stage 25, having a first input terminal, which is coupled to the output of the error-amplifier stage 22 and receives the control signal Vcon, a second input terminal, which is coupled to the second input 12b and receives the sense signal Vs, and an output terminal, which is coupled to the output 12c of the control circuit 12 and supplies the driving signal PW. In greater detail, the controller stage 25 comprises a comparator 28, designed to compare the control signal Vcon with the sense voltage Vs, and a PWM generator block 29, which is cascaded to the comparator 28 and is designed to generate the driving signal PW as a function of the result of the comparison.
There now follows a brief description of the general operation of the voltage converter 1 illustrated above.
Given the absence of an optocoupler between the secondary side of the transformer 4 and the control circuit 12, the value of the output voltage Vout is read from the auxiliary winding 7, via the resistive divider 10 upstream of the second rectifier diode 17. In the ideal case of absence of leakage inductances and of parasitic resistances of the transformer 4 and of the wires, and assuming the voltage drop on the first rectifier diode 9 to be negligible, the auxiliary voltage Vaus taken on the auxiliary winding 7 is proportional to the output voltage Vout during the period in which, between one switching cycle and the next, the first rectifier diode 9 is in conduction, basically for the entire duration of demagnetization of the transformer 4. In actual fact, due to the leakage inductances and the equivalent resistance on the secondary winding of the transformer 4, a damped oscillation is superimposed on the useful signal of the auxiliary voltage Vaus; this oscillation causes the auxiliary voltage Vaus to be a faithful replica, but for the turn ratio of the transformer 4, of the output voltage Vout only at the instant in which the demagnetization of the transformer 4 is concluded. In fact, in this instant of time the current on the secondary winding is zero, and hence the equivalent resistance on the secondary winding has no effect, and moreover the oscillations due to the leakage inductances have ended (assuming that the demagnetization time is sufficiently long). The plot of the output signal Vout and of the auxiliary voltage Vaus is shown in FIG. 2a, where the demagnetization period is designated by Tdem. FIG. 2b shows the corresponding plot of the demagnetization current Idem, which goes to zero at the end of the demagnetization period Tdem.
The sampling stage 20 is consequently configured to sample the feedback signal Vfb exactly at the instant of demagnetization of the transformer 4, in such a way that the sampled signal Vcam will coincide, but for the turn ratio of the transformer 4 and the dividing ratio of the resistive divider 10, with the output voltage Vout.
The difference between the reference signal Vref, which represents the value of the output voltage to be regulated, and the sampled signal Vcam constitutes the error signal Ve at input to the error-amplifier stage 22. In addition, the output of the error-amplifier stage 22, appropriately compensated so as to obtain the desired closed-loop transfer function, constitutes the signal, which, at input to the controller stage 25 determines the current peak on the primary winding, and hence the switching-on time of the power switch 8 (in PWM mode). In particular, the controller stage 25 charges the magnetization inductance of the transformer 4 with an energy proportional to the square of the peak current.
The main limit of the system for regulation of the output voltage Vout described above is represented by the difficulty of ensuring a same effectiveness of regulation in a wide range of values of the output current, and an adequate response to the load transients. In particular, a correct regulation of the output voltage Vout is impaired by the error inevitably present on the sampled signal Vcam especially at low loads, as a consequence of sampling occurring in the presence of the aforementioned oscillatory phenomenon.